Renames sidebar -> side-bar, sidebar/sidebar -> left-bar, rightbar -> right-bar

This PR is *not* ready to be merged. The changes *should* be good, but it will break 95% of plugins, including Kodowa plugins (see: https://groups.google.com/d/msg/light-table-discussion/D6-aqBIfPFQ/fp7DRi6Wgv4J ).

This PR is put up for two reasons:
1. Review for code correctness, since it wasn't a trivial remapping due to sidebar having different meanings depending on context; and
2. Request for suggestions on how best to add support for using the previous names (with a deprecation warning) as a transition period in the next major release, rather than needing to jump shark directly to the new names.